1. Field of the Invention
The present invention relates to an optical system in a lithographic apparatus and a method for manufacturing a device.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus may be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which may be referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern may be transferred on (part of) the substrate (e.g. a glass plate), e.g. via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
Instead of a circuit pattern, the patterning device may be used to generate other patterns, for example a color filter pattern, or a matrix of dots. Instead of a mask, the patterning device may include a patterning array that includes an array of individually controllable elements. An advantage of such a system compared to a mask-based system is that the pattern can be changed more quickly and for less cost.
A flat panel display substrate may be rectangular in shape. Lithographic apparatus designed to expose a substrate of this type may provide an exposure region which covers a full width of the rectangular substrate, or which covers a portion of the width (for example half of the width). The substrate may be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through the beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure may be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate may be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
The aim of lithography is to achieve more and smaller patterns on substrates in order to make smaller devices such as ICs. A difficulty lies in focusing the radiation onto a patterning device, and then focusing the patterned radiation onto the increasingly small and more densely packed target area of the substrate. This is particularly difficult with short wavelength radiation such as X-rays, EUV or DUV (with typical wavelengths such as 248 or 193 nm), that cannot be focused enough using traditional lenses used for visible light and the like. Furthermore, there are several areas in lithography that require efficient and delicate focusing of radiation that traditional methods do not achieve. Traditionally, focusing elements have included glass lenses. However, glass lenses have a relatively large cross sectional and surface area and so the layout of glass lenses is limited. Furthermore, glass has a specific lifetime and may be prone to chipping or clouding. Furthermore, low wavelength radiation (such as X-rays) does not transmit or refract through glass.